Estimating emotional responses to pictures based on heart rate measurements: Variations in Heart Rate serves as an important clinical health indicator, but potentially also as a window into cognitive reactions to presented stimuli, as a function of both stimuli, context and previous cognitive state. This study looks at single-trial time domain mean Heart Rate (HR) and frequency domain Heart Rate Variability (HRV) measured while subjects were passively viewing emotionally engaging images, comparing short random presentations with grouped sequences of either neutral, highly arousing pleasant or highly arousing unpleasant pictures. For the grouped sequences, we found a trend in the mean HR that could correlate with the emotional content of the images, but no such trends was seen in the random trials. We were, however, not able to demonstrate HRV variations that correlated with the presented emotional content, nor could we reproduce earlier studies, with different experimental setups that were based on average values over many subjects, that had revealed small changes in the mean HR only seconds after presentation.